This invention relates to an interface for interfacing between two different types of networks, such as a GSM cellular radio network and an integrated services digital network (ISDN).
This is a continuation of international patent application Ser. No. PCT/EP91/00853 filed on May 6, 1991 in the European Patent Office and designating the United States, now abandoned.
An ISDN network provides communications on two traffic channels (usually labelled B1 and B2) and a control channel (usually referred to as the D channel). The bandwidth of the traffic channels makes them capable of carrying 64 kB/s per channel and the control channel is capable of carrying up to 16 kB/s.
The GSM (Groupe Speciale Mobile) cellular radio system provides communications on a traffic channel carrying 13 kB/s of speech or 12 or 9.6 kB/s of data. The GSM system provides a control channel of up to 1 kB/s.
It is desirable in the manufacture of GSM equipment to provide an interface (usually referred to as the S interface) for connection of ISDN compatible equipment. Thus, for example, an ISDN telephone, fax, answering machine or other such equipment can be plugged into a GSM mobile transceiver and operated from a vehicle or other remote GSM subscriber equipment.
Manufacturers of ISDN compatible equipment propose to provide the facility of transmitting packets of data on the D channel. For example, a number of ISDN telephones could be installed on a fixed network and a supervision device might transmit data on the D channel. The devices could initiate a connection to a supervisor computer without initiating a call. ISDN service operators might charge lower fees for transmitting a packet of data on the D channel, rather than establishing a point-to-point communication on the B2 channel. Other uses for data on the D channel would include credit card checking, supervision and alarm systems, broadcast message services etc.
It is a problem with connection of ISDN equipment to GSM equipment that the GSM control channel does not have the capacity for supporting data from the ISDN control channel.